Add swift-configuration support

[adam-fowler]

Add MQTTConnection.withConnection(configReader: ConfigReader, ...)
Add MQTTConnectionConfiguration(configReader: ConfigReader)

[fpseverino]

There are a few decisions we need to make in order to implement it:

• There are subtypes of MQTTConnectionConfiguration, such as VersionConfiguration and TLS which are basically impossible to get from a ConfigReader. PingConfiguration would also be weird. In STOMP NIO I initialized similarly complex types with their default value. The user could set them later like this:

var config = MQTTConnectionConfiguration(configReader: ...)
config.versionConfiguration = .v5_0(...)

• In STOMP NIO I scoped every key like this: stomp.auth.login, stomp.virtualHost, stomp.webSocket.urlPath, etc. The Swift Configuration guide doesn't add this sort of namespace, so they would be like auth.login, and so on. To be fair ConfigReader has methods to make a copy of it with the "namespace" that you can then pass to the init. What do you say? Scope or no scope?
• You proposed .init(configReader: ConfigReader), the Swift Configuration guide (and STOMP NIO, for what it's worth) use .init(config: ConfigReader). WDYT?

[adam-fowler]

There are a few decisions we need to make in order to implement it:

• There are subtypes of MQTTConnectionConfiguration, such as VersionConfiguration and TLS which are basically impossible to get from a ConfigReader. PingConfiguration would also be weird. In STOMP NIO I initialized similarly complex types with their default value. The user could set them later like this:

• Version I guess would be defined by version = "3.1.1" or "5.0", or based on the existence of v3_1_1 or v5.
• Version configuration can be done with variables such as v3_1_1.will.topic, v5.will.topic. The MQTT properties would be a lot harder and we can probably skip those.
• For TLS in HB I implemented the simple cases using PEM text. See https://github.com/hummingbird-project/hummingbird/blob/a1edb9d909dcd49fa3d11f86688f639db580b886/Sources/HummingbirdTLS/TLSChannel%2BConfigReader.swift#L35
• Ping configuration could be ping == "useServerKeepAlive", "disable" or a number.

var config = MQTTConnectionConfiguration(configReader: ...)
config.versionConfiguration = .v5_0(...)

• In STOMP NIO I scoped every key like this: stomp.auth.login, stomp.virtualHost, stomp.webSocket.urlPath, etc. The Swift Configuration guide doesn't add this sort of namespace, so they would be like auth.login, and so on. To be fair ConfigReader has methods to make a copy of it with the "namespace" that you can then pass to the init. What do you say? Scope or no scope?

No scope. People should can the function MQTTConnection(config: config.scoped(to: "mqtt")). They might have multiple MQTT servers they are connecting to with different configurations, so want different scoping for each.

• You proposed .init(configReader: ConfigReader), the Swift Configuration guide (and STOMP NIO, for what it's worth) use .init(config: ConfigReader). WDYT?

Yeah .init(config:) is fine